Insight Into an Ancient Plate Boundary: A Structural Investigation of the Granite Wash Mountains (Arizona, U.S.A.)

Abstract

The Maria fold-and-thrust belt in the Mojave-Sonoran deserts of SE California and west-central Arizona preserves multi-deformed rocks involved in Jurassic (ca. 165-140 Ma), Cretaceous (ca. 120-100 Ma) and Cretaceous-Eocene ( ca. 80-40 Ma) tectonic events that affected southwestern North America. An ongoing debate remains on the plate organization and geodynamics of the western North American margin during the Late Cretaceous through the Eocene. In the Granite Wash Mountains (AZ), thrust faults associated with Cretaceous crustal shortening are overprinted and cross-cut by ductile shear zones. Shear zones strike dominantly NW-SE, mirroring Miocene (ca. 20-15 Ma) low-angle detachment faults in the region and raising questions as to their genetic relationships with these structures. In the southern Granite Wash and Little Harquahala ranges, metasedimentary fabrics co-located with the Cretaceous-aged Hercules Thrust system are cross-cut by syn-kinematic dykes that show map-scale s-fabrics, suggesting possible NW-SE transtensional motion. The Tank Pass Granite pluton (ca. 70 Ma), previously thought to have been undeformed, preserves evidence of both top-NE and top-SW shearing and macroscale S-C fabrics, which we present here with new fine-scale geologic mapping, field relationships, and stereograms. These structural relationships between shear zones and their host materials lay the foundation for future work targeting the temperature conditions via microstructural analysis and ages of deformation preserved in these shear zones